How to use functions in c++? - c++

Question

I am supposed to get the following code to display something along the lines of: "The sum of 1 to 10 is 55." (The larger number can be any number that was just the example I got.) I was given this code to use.
#include <iostream>
using namespace std;
// Compute the sum of all of the numbers from 1 to n where n
// is a natural number
// use the formula: n(n+1)/2
void compute_sum(int limit) // compute_sum function
{
int sum_to_limit;
sum_to_limit = limit * (limit + 1) / 2;
}
int main()
{
int sum = 0;
int maxNumber;
// get the maxNumber for the function call
cout << "Enter a whole number greater than 0" << endl;
cin >> maxNumber;
// call compute sum
compute_sum(maxNumber); // Call to compute_sum function
// display the sum calculated by the compute_sum function
cout << "The sum of 1 to " << maxNumber;
cout << " is " << sum << endl;
return 0;
}
I do not understand how funcctions work at all and do not have any idea how I would go about getting this to work. The only thing I know about this (and this is from the teacher) is that the change required is not major. "Note: If you are making major changes to the main and compute_sum funtions you are probably
doing way too much work." I have tried changing the function to an int function with a return but I could not get it to work properly (most likely due to not knowing how functions work). So can someone please help me?

Answer 1

The part you're missing is the return type of the function, and then to actually return that value from the function.
At the moment you have
void compute_sum(int limit) // compute_sum function
{
int sum_to_limit;
sum_to_limit = limit * (limit + 1) / 2;
}
A function prototype in C looks pretty much like this
<return type> <name> (<parameters>)
{
// your logic here
return <your_own_variable> // Note: You can omit this if the return type is void (it means the function doesn't return anything)
}
You want to modify your function so you are returning the integer value you're calculating inside of it
int compute_sum(int limit) // compute_sum function
{
int sum_to_limit;
sum_to_limit = limit * (limit + 1) / 2;
return sum_to_limit;
}
So what happens is after main runs, when the the point of execution hits
compute_sum(maxNumber);
The program flow jumps to that function and executes the code inside of it. When the function finishes, it returns the value back to where it was originally called from. So you also need to add this to store the value returned
int result = compute_sum(maxNumber);
and then make sure to output that value to the user.
You can also make the computer_sum function a little more terse my not storing a temporary variable, you can just do this
int compute_sum(int limit) // compute_sum function
{
return limit * (limit + 1) / 2;
}
I hope that helps. There's a lot more going on behind the scenes but that's the basic idea. Good luck! :)